The present invention is directed to a mobile ad hoc network (MANET). More specifically, the present invention is directed to a method and system for limiting interference between mobile units communicating in a MANET.
A MANET is basically a network of mobile wireless terminals, such as wireless phones, that communicate with each other within a given region or area. Each wireless terminal is capable of receiving/transmitting data packets to/from other wireless terminals in the network. The wireless terminals have a detection range (i.e. the distance within which a wireless terminal can detect a transmission from another wireless terminal) and a link range (i.e. the maximum distance apart two wireless terminals may be and yet still send data successfully from one to the other in the absence of interference), and an interference range (i.e. the maximum distance apart two wireless terminals A and B may be such that there exists a point P that is within link range of A and yet a transmission from B would interfere with the transfer of data from A to a wireless terminal located at P). A wireless terminal B is said to interfere with a transmission from terminal A to wireless terminal C if, in the absence of terminal B, terminal C would successfully receive the transmission from terminal A, and if terminal B transmits during the transmission from terminal A to terminal C it results in an unsuccessful reception of the transmission at terminal C.
The wireless terminals basically collaborate to route packets amongst themselves according to link relationships. Current approaches to MANET use Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) techniques to control the broadcasting of the wireless terminals. In CSMA/CA, a wireless terminal briefly listens for the “carrier” presence. If it is present, the wireless terminal waits to transmit/broadcast. If it is not present, the wireless terminal transmits/broadcasts immediately.
In a MANET environment, the difficulty with CSMA/CA is that communications between a sending wireless terminal and a receiving wireless terminal may be lost when the sending wireless terminal is outside the detection range of a distant wireless terminal, and the receiving wireless terminal is within the interference range of the distant wireless terminal. When this happens, the sending wireless terminal will not detect the carrier of the distant wireless terminal and, in accordance with CSMA/CA, will transmit to the receiving wireless terminal. The transmission, however, will not be received by the receiving wireless terminal because its reception will be interfered with by the distant wireless terminal. Thus, broadcasting within a MANET can be unreliable due to such collision/interference from distant wireless terminals.